Hypersensitivity pneumonitis (HP) is an inflammatory lung disease that develops following repeated exposure to inhaled particulate antigen. The epidemiology of HP remains largely unknown. HP is characterized by a vigorous Th1 (or type 1) mediated immune response and previous studies suggest that Th1 cytokines such as IL-12 and IFN-y play an important role in the pathogenesis of HP. Stachybotrys chartarum (SC) is a dimorphic fungus that has been implicated in a number of respiratory illnesses, including sick building syndrome, asthma, and HP. In preliminary studies, we have found that inhalation of SC conidia in sensitized mice results in the development of a pulmonary hypersensitivity response. Moreover, granulmatous inflammation induced by SC is substantially reduced in mice deficient in the intracellular toll- like receptor, TLR9. Mechanisms accounting for the altered granulomatous response observed in TLR9- deficient mice have not been defined and are the focus of this studies outlined in this proposal. Based on these preliminary studies, we hypothesize that the dendritic cell (DC) is the essential antigen presenting cell that promotes Th1 responses during the development of SC-induced HP, and the promotion of granulomatous inflammation by DC is mediated by TLR9. The following Specific Aims are designed to test this hypothesis: Aim 1) To determine the effect of SC conidia on DC effector responses in vitro, and determine if TLR9 is required for SC-mediated DC activation; Aim 2) To determine the contribution of TLR9 to SC-induced pulmonary granulomatous inflammation in vivo; Aim 3) To determine the contribution of TLR9-mediated DC responses in the generation of granulamatous inflammation in SC-induced HP. This proposal will serve as a natural extension of work I have performed to date investigating TLR9- mediated responses in lung antibacterial immunity. The training program outlined has been designed to provide the necessary techniques and knowledge base to successfully complete the studies outlined and the tools required to develop into a successful independent physician-scientist. RELEVANCE (See instructions): Hypersensitivity pneumontis is a complex syndrome of varying intensity and clinical presentation affecting approximately 9% of the general population. Mechanisms underlying cause and progression of the disease are still unknown. This project will provide mechanistic insight into the role of TLR9 mediated dendritic cell responses on the generation of granulomatous inflammation.